Fix DockerEnvironment unit tests by removing eager Docker dependency

[AgentDosaku]

Summary

• make DockerEnvironment create its Docker client lazily (in setup())
• add optional client_factory constructor arg for testability/injection
• keep runtime behavior the same for normal usage

Why

Unit tests should not require a running Docker daemon. Previously, DockerEnvironment.init eagerly called docker.from_env(), causing unit tests to fail in environments without Docker.

Validation

• pytest -q tests/unit/mindtrace/cluster/test_docker_env.py (15 passed)
• ds test --unit (4174 passed, 14 skipped)